Abstract:

The present invention provide a high-luminosity stress-stimulated
luminescent material which emits visible light even in daylight, a
manufacturing method thereof, and a typical example of the use thereof.
The stress-stimulated luminescent material of the present invention
satisfies conditions for light emission by at least one of: a
luminescence mechanism using static electricity caused by friction; a
luminescence mechanism using micro plasma caused by friction; a
luminescence mechanism using a piezoelectric effect caused by strain; a
luminescence mechanism using lattice defect; and a luminescence mechanism
using thermal generation. For example, in case where a base material made
of at least one type of aluminate is includes as the stress-stimulated
luminescent material, the base material includes a crystal structure with
spontaneous polarization, e.g. α-SrAl2O4, in order to
realize the luminescence mechanism using the piezoelectric effect caused
by strain.

Claims:

1. A stress-stimulated luminescent material which emits light in response
to a mechanical effect,the stress-stimulated luminescent material which
is characterized by satisfying a condition to emit light based on at
least one of: a luminescence mechanism using static electricity caused by
friction; a luminescence mechanism using micro plasma caused by friction;
a luminescence mechanism using a piezoelectric effect caused by
mechanical strain; a luminescence mechanism using lattice defect; and a
luminescence mechanism using thermal generation.

2. The stress-stimulated luminescent material as defined in claim 1,
further comprising a base material constituted by at least one type of
aluminate,the base material including a crystal structure having
spontaneous polarization, in order to realize the luminescence mechanism
using the piezoelectric effect caused by strain, or the like.

3. The stress-stimulated luminescent material as defined in claim
2,wherein, the base material is α-SrAl2O.sub.4.

4. The stress-stimulated luminescent material as defined in claim
2,wherein, to realize the luminescence mechanism using the lattice
defect, at least two types of metal ions are added, as central ions of
defect center, to the base material.

5. The stress-stimulated luminescent material as defined in claim
4,wherein, as a result of the addition of the central ions, the lattice
defect is formed in the crystal structure having the spontaneous
polarization in the base material.

6. The stress-stimulated luminescent material as defined in claim
4,wherein, the crystal structure has a tunnel structure, and an element
is provided in the tunnel by ionic bond.

7. The stress-stimulated luminescent material as defined in claim
4,wherein, the added central ions substitute Sr sites of the
α-SrAl2O.sub.4.

8. The stress-stimulated luminescent material as defined in claim
7,wherein, the metal ions added as the central ions are smaller in ion
diameter than the Sr.

9. The stress-stimulated luminescent material as defined in claim
8,wherein, the metal ions which are smaller in ion diameter than the Sr
are an element selected from the group consisting of Mg, Na, Zn, Cu, Eu,
Tm, Ho, Dy, Sn, Mn, Nd, Pr, and Ca.

10. The stress-stimulated luminescent material as defined in claim
7,wherein, the metal ions added as the central ions are larger in ion
diameter than the Sr.

11. The stress-stimulated luminescent material as defined in claim
10,wherein, the metal ions which are larger in ion diameter than the Sr
are Ba and/or K.

12. The stress-stimulated luminescent material as defined in claim
7,wherein, the central ions are both larger and smaller in ion diameter
than the Sr.

13. The stress-stimulated luminescent material as defined in claim
7,wherein, the metal ions which are added as the central ions and
substitute the Sr sites of the α-SrAl2O4 are 0.1 to 40
mol % of the Sr.

14. The stress-stimulated luminescent material as defined in claim
7,wherein, in case where metal ions which are smaller in ion diameter
than the Sr and metal ions which are larger in ion diameter than the Sr
are both added as the central ions, an amount of the metal ions in total
is smaller than stoichiometry.

15. The stress-stimulated luminescent material as defined in claim
4,wherein, the added central ions substitute the Al sites of the
α-SrAl2O.sub.4.

16. The stress-stimulated luminescent material as defined in claim
15,wherein, the metal ions added as the central ions are smaller in ion
diameter than Al.

17. The stress-stimulated luminescent material as defined in claim
16,wherein, the metal ions smaller in ion diameter than the Al are Si or
B.

18. The stress-stimulated luminescent material as defined in claim
15,wherein, the metal ions added as the central ions are larger in ion
diameter than the Al.

19. The stress-stimulated luminescent material as defined in claim
18,wherein, the metal ions which are larger in ion diameter than the Al
are Ga or In.

20. The stress-stimulated luminescent material as defined in claim
15,wherein, the metal ions which are added as the central ions and
substitute the Al sites of the α-SrAl2O4 are 0.1 to 20
mol % of the Al.

21. The stress-stimulated luminescent material as defined in claim
4,wherein, the metal ions added as the central ions are made up of two or
more types of metal ions having different valence.

22. The stress-stimulated luminescent material as defined in claim
2,wherein, luminescence is in proportion to a strain energy density of
the material.

23. The stress-stimulated luminescent material as defined in claim 1,
further comprising a base material constituted by at least one type of
aluminate,the base material including a structure having symmetrical
center, in order to realize the luminescence mechanism using the static
electricity and micro plasma caused by the friction.

24. The stress-stimulated luminescent material as defined in claim
23,wherein, to realize the luminescence mechanism using the lattice
defect, at least one type of ions is added, as central ions of defect
center, to the base material.

25. The stress-stimulated luminescent material as defined in claim
23,wherein, the base material is a spinel-structured material with a
Zn--Al--O defect structure.

26. The stress-stimulated luminescent material as defined in claim
25,wherein, the base material is ZnAl2O4:Mn.

27. The stress-stimulated luminescent material as defined in claim
23,wherein, the base material is subjected to reduction at a temperature
range in which the lattice defect is formed.

28. A manufacturing method of a stress-stimulated luminescent material
which emits light in response to a mechanical stress,the method being
characterized by controlling a structure to satisfy a condition to emit
light based on at least one of: a luminescence mechanism using static
electricity caused by friction; a luminescence mechanism using micro
plasma caused by friction; a luminescence mechanism using a piezoelectric
effect caused by strain a luminescence mechanism using lattice defect;
and a luminescence mechanism using thermal generation.

29. The manufacturing method as defined in claim 28,wherein, to realize
the luminescence mechanism using the piezoelectric effect caused by
strain, raw materials are mixed and burned in such a manner as to form,
in a base material included in the stress-stimulated luminescent
material, a crystal structure with spontaneous polarization.

30. The stress-stimulated luminescent material as defined in claim
28,wherein, to realize the luminescence mechanism using the lattice
defect, at least one type of metal ions is added, as central ions of
defect center, to the base material in the stress-stimulated luminescent
material.

31. The manufacturing method as defined in claim 28,wherein, to realize
the luminescence mechanism using the static electricity and micro plasma
caused by strain, raw materials are mixed and burned in such a manner as
to form, in the base material included in the stress-stimulated
luminescent material, a structure having symmetrical center.

32. The manufacturing method as defined in claim 28,wherein, to realize
the luminescence mechanism using the thermal generation, raw materials
are mixed and burned in such a manner as to cause thermo-luminescence of
the base material in the stress-stimulated luminescent material to peak
around a use temperature of the stress-stimulated luminescent material.

33. A stress-stimulated luminescent body formed by molding the
stress-stimulated luminescent material defined in claim 1.

34. A stress-stimulated luminescent body formed by mixing the
stress-stimulated luminescent material defined in claim 1 with a
polymeric material and molding the stress-stimulated luminescent material
and the polymeric material into a flat plate.

35. A stress-stimulated luminescent body having a laminated structure in
which the stress-stimulated luminescent material defined in claim 1 is
provided on a supporting body.

36. The stress-stimulated luminescent body as defined in claim 35, having
a diaphragm structure.

37. A luminescence method of a stress-stimulated luminescent material,
characterized by applying an external force to the stress-stimulated
luminescent body defined in claim 33, the external force changing over
time.

38. The luminescence method as defined in claim 37,wherein, ultraviolet
light is applied to the stress-stimulated luminescent body while the
external force is applied over time.

39. A luminescence method of a stress-stimulated luminescent material,
characterized by rubbing the stress-stimulated luminescent material
define in claim 33, using a friction material.

40. The luminescence method as defined in claim 39,wherein, the friction
material has volume resistivity of not less than 10.sup.14 Ωcm at
25.degree. C. and 50% RH.

41. The luminescence method as defined in claim 40,wherein, the friction
material is made of polyethylene.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a stress-stimulated luminescent
material which emits light in response to mechanical stress, a
manufacturing method thereof, and the use thereof. The present invention
particularly relates to a stress-stimulated luminescent material which is
highly luminous and has a broader range of applications, a manufacturing
method thereof, and a typical example of the use of the stress-stimulated
luminescent material, such as a light emission method by which the
stress-stimulated luminescent material effectively emits light.

BACKGROUND ART

[0002]Stress-stimulated luminescent materials which emits light in
response to internal stress caused by a mechanical effect such as
friction, shearing, impact, and vibration have conventionally been
proposed. Known examples of the base material of such stress-stimulated
luminescent materials include aluminate, silicate, and semiconductor
found in nature. More specifically, for example, the inventors of the
present invention have proposed (1) a high-luminosity stress-stimulated
luminescent material made up of at least one type of aluminate of a
non-stoichiometric composition and including a material which includes
lattice defect causing luminescence when a carrier excited by mechanical
energy returns to the ground state. A specific example of such a material
is MxAl2O3+x (where M is Mg, Ca, Sr, or Ba, and
0.8<x<1). (For example, see Document 1: Japanese Laid-Open Patent
Application No. 2001-49251; published on Feb. 20, 2001.)

[0003]The inventors have also proposed (2) a stress-stimulated luminescent
material whose base material is an oxide constituted by a compound
represented as MN2O4 (where M and N are metal elements selected
from a group of Mg, Sr, Ba, and Zn and a group of Ga and Al,
respectively). (See, for example, Document 2: Japanese Laid-Open Patent
Application No. 2002-194349; published on Jul. 10, 2002).

[0004]The inventors have also proposed, as an example of a manufacturing
method of a high-luminosity stress-stimulated luminescent material, a
method of manufacturing aluminate by a sol-gel process, the aluminate
being represented by a general formula MxAlyO2x+3y/2
(where M is an alkaline earth metal, a transition metal, or a rare-earth
metal, x and y are integers, and the alkaline earth metal as M is a metal
element selected from a group of Mg, Ca, Ba, and Sr). (See, for example,
Japanese Laid-Open Patent Application No. 2002-220587; published on Aug.
9, 2002).

[0005]The conventional stress-stimulated luminescent materials can
effectively emit light by mechanical stress, but the light emission
luminescence is sometimes insufficient. The applications and uses of the
stress-stimulated luminescent materials are therefore limited.

[0006]More specifically, under a circumstance where the light intensity of
outside light is relatively low, the above-described stress-stimulated
luminescent materials can perform stress-stimulated luminescence with an
intensity sufficient for visual observation. On the other hand, in case
where the light intensity of outside light is as high as daylight, i.e.
under a fairly bright circumstance, the light emission luminescence of
the stress-stimulated luminescent material may be relatively low and not
sufficient for visual observation.

[0007]Strong luminous intensity sufficient for visual observation under a
bright circumstance is required to broaden the range of uses and
applications of luminescent materials. A stress-stimulated luminescent
material which can achieve such strong light emission cannot be
practically obtained by conventional techniques.

DISCLOSURE OF INVENTION

[0008]The invention was done to solve the above-described problem, and the
objective of the present invention is to provide a high-luminosity
stress-stimulated luminescent material which emits visible light even in
daylight, a manufacturing method thereof, and a typical example of the
use thereof.

[0009]As a result of the diligent research on the problem, by clarifying
the luminescence mechanism of the stress-stimulated luminescent material,
the inventors independently found that stress-stimulated luminescence
with a luminous intensity higher than ever could be realized. Also, the
inventors completed the invention by finding out an effective
luminescence method of the stress-stimulated luminescent material, based
on the clarified luminescence mechanism.

[0010]That is, the stress-stimulated luminescence material of the present
invention emits light in response to a mechanical effect, the
stress-stimulated luminescent material being characterized by satisfying
a condition to emit light based on at least one of: a luminescence
mechanism using static electricity caused by friction; a luminescence
mechanism using micro plasma caused by friction; a luminescence mechanism
using a piezoelectric effect caused by strain; a luminescence mechanism
using lattice defect; and a luminescence mechanism using thermal
generation.

[0011]An example of the above-described stress-stimulated luminescent
material further includes a base material constituted by at least one
type of aluminate, the base material including a crystal structure having
spontaneous polarization, in order to realize the luminescence mechanism
using the piezoelectric effect caused by strain.

[0012]The most preferable example of the above-described stress-stimulated
luminescent material is arranged such that the base material is
α-SrAl2O4.

[0013]The above-described stress-stimulated luminescent material is
preferably arranged such that, to realize the luminescence mechanism
using the lattice defect, at least two types of metal ions are added, as
central ions of defect center, to the base material. As a result of the
addition of the central ions, the lattice defect is formed in the crystal
structure having the spontaneous polarization in the base material. Also,
in the stress-stimulated luminescent material, the crystal structure has
a tunnel structure, and an element is provided in the tunnel by ionic
bond. In other words, the central ions are provided in the tunnel.

[0014]More specifically, the added central ions substitute Sr sites of the
α-SrAl2O4, for example. In this case, if the metal ions
added as the central ions are smaller in ion diameter than the Sr, the
metal ions may be an element selected from the group consisting of Mg,
Na, Zn, Cu, Eu, Tm, Ho, Dy, Sn, Mn, Nd, Pr, and Ca. On the other hand, if
the metal ions added as the central ions are larger in ion diameter than
the Sr, the metal ions may be Ba and/or K.

[0015]It is particularly preferable that the central ions are both larger
and smaller in ion diameter than the Sr. It is also preferable that the
total amount of the metal ions which are added as the central ions and
substitute the Sr sites of the α-SrAl2O4 is 0.1 to 40 mol
% of the Sr, and more preferably 5-25 mol % of the Sr. In case where the
added central ions are both larger and smaller in ion diameter than the
Sr, the total amount of the metal ions is smaller than stoichiometry.

[0016]The stress-stimulated luminescent material of the present invention
may be arranged such that the added central ions substitute the Al sites
of the α-SrAl2O4. In this case, if the ion diameter of
the metal ions added as the central ions is shorter than the diameter of
the Al, the metal ions are preferably Si or B, for example. If the ion
diameter of the metal ions added as the central ions is longer than the
diameter of the Al, the metal ions are preferably Ga or In, for example.
The metal ions added as the central ions and substituting Al sites of the
α-SrAl2O4 is preferably 0.1 to 20 mol % of the Al.

[0017]The above-described stress-stimulated luminescent material is
preferably arranged such that the metal ions added as the central ions
are made up of two or more types of metal ions having different valence.
For example, positive univalent metal ions and positive bivalent metal
ions may be simultaneously added, positive bivalent metal ions and
positive tervalent metal ions may be simultaneously added, or univalent
metal ions, positive bivalent metal ions, and positive tervalent metal
ions may be simultaneously added.

[0018]In the stress-stimulated luminescent material in which the base
material has a crystal structure with spontaneous polarization, the
luminescence is in proportion to the strain energy of the material.

[0019]Another example of the stress-stimulated luminescent material
includes a base material constituted by at least one type of aluminate,
the base material including a structure having symmetrical center, in
order to realize the luminescence mechanism using the static electricity
and micro plasma caused by the friction.

[0020]The above-described stress-stimulated luminescent material is
preferably arranged such that, to realize the luminescence mechanism
using the lattice defect, at least one type of ions is added, as central
ions of defect center, to the base material.

[0021]A specific example of the stress-stimulated luminescent material is
arranged such that the base material is a spinel-structured material with
a Zn--Al--O defect structure, and more specifically, the base material is
ZnAl2O4:Mn. It is preferable that the base material is
subjected to reduction at a temperature range in which the lattice defect
is formed.

[0022]A manufacturing method of a stress-stimulated luminescent material,
which emits light in response to a mechanical stress, is characterized by
controlling a structure to satisfy a condition to emit light based on at
least one of: a luminescence mechanism using static electricity caused by
friction; a luminescence mechanism using micro plasma caused by friction;
a luminescence mechanism using a piezoelectric effect caused by strain a
luminescence mechanism using lattice defect; and a luminescence mechanism
using thermal generation.

[0023]The above-described manufacturing method is preferably arranged such
that, to realize the luminescence mechanism using the piezoelectric
effect caused by strain, raw materials are mixed and burned in such a
manner as to form, in a base material included in the stress-stimulated
luminescent material, a crystal structure with spontaneous polarization.
It is also preferable that, to realize the luminescence mechanism using
the lattice defect, at least one type of metal ions is added, as central
ions of defect center, to the base material in the stress-stimulated
luminescent material. It is also preferable that, to realize the
luminescence mechanism using the static electricity and micro plasma
caused by strain, raw materials are mixed and burned in such a manner as
to form, in the base material included in the stress-stimulated
luminescent material, a structure having symmetrical center. It is also
preferable that, to realize the luminescence mechanism using the thermal
generation, raw materials are mixed and burned in such a manner as to
cause thermo-luminescence of the base material in the stress-stimulated
luminescent material to peak around a use temperature of the
stress-stimulated luminescent material. The thermo-luminescence
preferably has plural peaks. It is also preferable that the
thermo-luminescence show plural peaks around a use temperature in range
of 100° C.

[0024]Non-limiting examples of the use of the present invention are a
stress-stimulated luminescent body formed by molding the above-described
stress-stimulated luminescent material, and a stress-stimulated
luminescent body formed by mixing the above-described stress-stimulated
luminescent material with a polymeric material and molding them into a
flat plate. The method and form of the molding are not particularly
limited. In particular, in case where the stress-stimulated luminescent
material including a crystal structure with spontaneous polarization is
used, a stress-stimulated luminescent body including a laminated
structure in which the above-described stress-stimulated luminescent
material is provided on a supporting body is preferable. In this case, it
is more preferable to include a diaphragm structure.

[0025]Another example of the use of the present invention is a
luminescence method of a stress-stimulated luminescent material, which is
characterized in that characterized by applying an external force to the
above-described stress-stimulated luminescent body, the external force
changing over time. In this case it is preferable that ultraviolet light
is applied while the external force changing overtime is applied to the
stress-stimulated luminescent body. Also, in the case of a
stress-stimulated luminescent material having the luminescence mechanism
using friction, the stress-stimulated luminescent material is preferably
rubbed by a friction material.

[0026]Non-limiting example of the friction material is a material having a
high electric resistance. More specifically, a material having volume
resistivity of not less than 1014 Ωcm at 25° C. and 50%
RH is preferable. An example of such a material is polyethylene.

[0027]Additional objects, features, and strengths of the present invention
will be made clear by the description below. Further, the advantages of
the present invention will be evident from the following explanation in
reference to the drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0028]FIG. 1 is a schematic diagram showing the structure of
α-SrAl2O4 which is an example of a stress-stimulated
luminescent material of the present invention.

[0029]FIG. 2 is a schematic diagram illustrating the excitation mechanism
of electroluminescence in a friction luminescent material and the
occurrence of static electricity in the friction luminescent material, in
the stress-stimulated luminescent material of the present invention.

[0030]FIG. 3 is a chart showing the XRD pattern of the Zn--Al--O
stress-stimulated luminescent material of Example 1.

[0031]FIG. 4 is a graph illustrating the weight loss curve in case where
the Zn--Al--O stress-stimulated luminescent material is reduced.

[0032]FIG. 5 is a graph showing the fluorescence intensity of the
Zn--Al--O stress-stimulated luminescent material.

[0033]FIG. 6 is a chart showing the thermo-luminescence of the Zn--Al--O
stress-stimulated luminescent material.

[0034]FIG. 7 is a graph illustrating, in Example 1, to what extent the
thermo-luminescence of ZnAl2O4:Mn, which is an example of the
stress-stimulated luminescent material, depends on a reduction
temperature.

[0035]FIG. 8 is a graph showing, in Example 1, to what extent the
thermo-luminescence of ZnAl2O4:Mn, which is an example of the
stress-stimulated luminescent material, depends on reduction, and also
showing the response curve of friction luminescence.

[0036]FIG. 9 is a chart which relates to Example 1 and shows spectrums of
fluorescence (PL), thermo-luminescence (ThL), and friction luminescence
(ML) of the stress-stimulated luminescent material.

[0037]FIG. 10(a) is a graph showing the correlativity between the
reduction temperature and the friction luminous intensity in Example 1.

[0038]FIG. 10(b) is a graph showing, in Example 1, the relationship
between the afterglow intensity and friction luminous intensity of
ZnAl2O4:Mn after the reduction at 1200° C.

[0039]FIG. 11(a) is a graph illustrating the intensity of friction
luminescence of each friction material in Example 1.

[0040]FIG. 11(b) is a graph illustrating the surface potential of each
friction material in Example 1.

[0041]FIG. 12 is a chart illustrating an electroluminescent spectrum of
the stress-stimulated luminescent material in Example 1.

[0042]FIG. 13 is a schematic view showing an example of a measurement
system for measuring stress-stimulated luminescence of a sample, an
applied stress, or the like, in Examples 2-8.

[0043]FIG. 14(a) a graph showing the variation of stress-stimulated
luminous intensity in case where a α-SrAl2O4 phase
coexists with a non-luminescent crystal phase in Example 3.

[0044]FIG. 14(b) is a graph showing the variation of stress-stimulated
luminous intensity in case where the α-SrAl2O4 phase
coexists with a non-luminescent crystal phase in Example 3.

[0045]FIG. 15 is a graph showing the relationship between luminous
intensity and stress in case where the α-SrAl2O4 phase
coexists with another phase in Example 3.

[0046]FIG. 16 is a graph showing the spectrum of stress-stimulated
luminescence in case where the α-SrAl2O4 phase coexists
with another phase in Example 3.

[0047]FIG. 17 shows the relationship between luminous intensity and
lattice defect density in α-SrAl2O4 phase in case where
lattice defect is formed in Sr sites in Example 4.

[0048]FIG. 18 is a chart showing an example of thermo-luminescence of the
stress-stimulated luminescent material which has high stress-stimulated
luminous intensity in Example 5.

[0049]FIG. 19 is a chart showing the luminescence variation curve in case
where a stress is applied to the stress-stimulated luminescent material
in Example 6.

[0050]FIG. 20 is a graph showing the correlativity between luminous
intensity and stress in the stress-stimulated luminescent material in
Example 6.

[0051]FIG. 21 is a graph showing the correlativity between luminous
intensity and strain speed in the stress-stimulated luminescent material
in Example 6.

[0052]FIG. 22 is a graph showing the variation of stress-stimulated
luminescence peaks when ultraviolet light irradiation is carried out and
not carried out, in case where a stress cycle of 1 Hz is applied.

[0053]FIG. 23 is a graph showing the variation of luminous intensity in
case where a stress cycle of 1 Hz is applied while ultraviolet light is
applied.

BEST MODE FOR CARRYING OUT THE INVENTION

[0054]The following will describe an embodiment of the present invention
in reference to FIGS. 1 and 2. It is noted that the present invention is
not limited to this embodiment. In the following, a luminescence
mechanism of stress-stimulated luminescent materials, by which the
present invention is completed, a stress-stimulated luminescent material
of the present invention and its manufacturing method, and the use of the
present invention will be discussed in this order, in a detailed manner.

[0056]Various types of high-luminosity stress-stimulated luminescent
materials have been proposed, and obtained luminescence characteristics
differ according to the types of applied external force. However, details
of the luminescence mechanism were not clear. In regard of this, the
inventors for the first time found out the luminescence mechanism by
clarifying the excitation source required for luminescence caused by a
mechanical effect. The inventors also found out the crystal structure,
composition, and effective luminescence method of the high-luminosity
stress-stimulated luminescent material.

[0057]More specifically, the present invention was done to manufacture a
stress-stimulated luminescent material which emits light based on any one
of: a luminescence mechanism using static electricity caused by friction;
a luminescence mechanism using micro plasma caused by friction; a
luminescence mechanism using a piezoelectric effect caused by strain; and
a luminescence mechanism using thermal generation.

[0058](I-1) Luminescence Mechanism of Luminescence by Friction

[0059]Luminescence of a stress-stimulated luminescent material which emits
light by friction is achieved by at least one of the following
mechanisms: generation of static electricity by friction; generation of
micro plasma by friction; existence of lattice defect; and thermal
generation by friction.

[0060]More specifically, luminescence is achieved in such a manner that
static electricity is generated by friction or the like, and local
electroluminescence is caused by the static electricity. Alternatively,
luminescence is achieved in such a manner that micro plasma is generated
because the surface atomic bond is broken by friction, and hence plasma
excitation occurs. Also, as described below, in case where lattice defect
exists, luminescence occurs in such a manner that electrons or positive
holes (holes) trapped in the lattice defect are released by friction and
the released electrons and holes are recombined. Also, as described
below, there is a mechanism in which light emission occurs because of
thermal generation by friction. In particular, materials with a structure
having symmetry center can easily achieve luminescence by friction, even
if the materials cannot easily achieve luminescence by strain. This is
because the crystal structure on the surface is asymmetric.

[0061]In other words, to construct a luminance mechanism of a
stress-stimulated luminescent material which emits light by friction, the
raw materials of the stress-stimulated luminescent material are mixed and
burned so that the base material of the stress-stimulated luminescent
material have a structure including symmetry center. In the present
invention, as described below, a spinel crystal material having a
Zn--Al--O defect structure is used as an example of a material which
emits intense light in response to friction excitation.

[0062](I-2) Luminescence Mechanism of Luminescence by Strain Energy

[0063]Luminescence of a stress-stimulated luminescent material which emits
light by strain energy is achieved by at least one of the following
mechanisms: a piezoelectric effect by strain energy; lattice defect by
strain energy; and thermal generation by deformation by strain energy.

[0064]Luminescence by a piezoelectric effect occurs in such a manner that
strain energy is generated in a material by application of a deforming
force, electricity is generated by a piezoelectric effect caused by the
strain energy, and hence electroluminescence occurs. The material which
emits light on account of a piezoelectric effect is therefore a
piezoelectric substance. On this account, the crystal structure is
required not to have symmetry center. Also, a material without symmetry
center is particularly required to have a structure in which spontaneous
polarization exists.

[0065]In other words, to construct the luminance mechanism of a
stress-stimulated luminescent material which emits light in response to a
piezoelectric effect by strain, the raw materials of the
stress-stimulated luminescent material are mixed and burned so that a
crystal structure having spontaneous polarization is formed in the base
material of the stress-stimulated luminescent material. The present
invention, as described below, takes a crystal material with a
α-SrAl2O4 phase as an example of a material which emits
intense light by a piezoelectric effect. As to the luminescence by
lattice defect and the luminescence by thermal generation, the next
chapter will deal with them because those types of luminescence are
similar to the luminescence by friction, as described above.

[0067]As described above, the luminescence mechanisms by lattice defect
and thermal generation can be realized in both luminescence by friction
and luminescence by deformation.

[0068]More specifically, in the case of luminescence by lattice defect,
electrons and positive holes (holes) trapped in the lattice defect of the
material on account of friction and strain energy are recombined, so that
luminescence is achieved. More specifically, when friction or strain
energy is applied, the crystal field around the center of the lattice
defect changes on account of the strain. This excites the trapped
electrons or positive holes, and these electrons and positive holes are
recombined. As a result, luminescence occurs. On this account, the site
where the lattice defect occurs preferably has a loosely-bound in the
crystal and is at a position susceptible to strain in the crystal. The
arrangement of the lattice defect in the material is therefore quite
important.

[0069]In other words, to achieve the luminance mechanism of a
stress-stimulated luminescent material which emits light by lattice
effect, at least one type, preferably more than one type of metal ions is
added, as central ions of defect center, to the base material in the
stress-stimulated luminescent material. As described below, in the
present invention, metals are added in a crystal material with a
α-SrAl2O4 phase, in such a manner that Sr sites and Al
sites are substituted by metal ions.

[0070]In the case of luminescence by thermal generation, a material
generates heat in response to friction, deformation causes the material
to deform, and hence the heat is generated with this deformation.
Luminescence by thermal generation is based on thermo-luminescence by the
thermal generation (temperature increase). In the luminescence mechanism
using thermo-luminescence, the position and form of the peak of the
thermo-luminescence are important. In consideration of the use of the
stress-stimulated luminescent material or the like, the peak of the
thermo-luminescence is preferably around the use temperature of the
stress-stimulated luminescent material. For example, in case where the
use temperature of the stress-stimulated luminescent material is around
room temperatures (15-25° C.), the peak of the thermo-luminescence
is preferably around this temperature range. More preferably, there are
plurality of peaks in the thermo-luminescence. It is also preferable that
the thermo-luminescence show plural peaks around a use temperature in
range of 100° C.

[0071]In other words, to construct the luminance mechanism of a
stress-stimulated luminescent material which emits light by thermal
generation, the raw materials of the stress-stimulated luminescent
material are mixed and burned in such a manner that the peak of the
thermo-luminescence of the base material in the stress-stimulated
luminescent material is around the use temperature of the
stress-stimulated luminescent material.

[0073]To summarize the chapter (I), there are at least four types of
luminescence mechanisms of a stress-stimulated luminescent material:
friction, piezoelectric effect by strain, lattice defect, and thermal
generation. It is possible to obtain a synergistic effect of the
luminescence mechanisms by materials which can adopt at least one type
of, preferably more than one type of, and more preferably all types of
the luminescence mechanisms. It is therefore possible to achieve
luminescence which is higher than ever. It is noted that luminescence by
lattice defect and thermal generation may be attenuated over time.

[0074]As a result of numerous experiments based on the above-described
idea, the inventors successfully developed high-luminosity
stress-stimulated luminescent materials. The following will describe, as
stress-stimulated luminescent materials of the present invention, the
most preferable examples of (1) a material which emits strong light by
friction and (2) a material which emits strong light by deformation.

[0075](II-1) Material Emitting Storing Light by Friction

[0076]Among the stress-stimulated luminescent materials of the present
invention, a material which emits light by friction (hereinafter, this
material will be referred to as friction-luminescent material for
convenience) performs stress-stimulated luminescence by at least one of:
(1) electroluminescence caused by static electricity generated by
friction or the like; (2) plasma luminescence caused by micro plasma
generated by friction; (3) luminescence by recombination of electrons and
positive holes trapped in lattice defect; and (4) thermo-luminescence
caused by heat generated by friction. When electroluminescence is caused,
the aforesaid material is regarded as a good electroluminescent material.
Since the friction-luminescent material emits light in response to
friction at the contact surface, the static electricity, the intensity of
the micro plasma, and the temperature variation are different according
to the material (friction material) with which the friction occurs. Also,
the static electricity, the intensity of the micro plasma, and the
temperature variation are different according to environments. These
properties may be degraded on account of water or moisture.

[0077]Although not particularly limited, the friction-luminescent material
of the present invention includes a base material made of at least one
type of aluminate, and the base material includes a structure having
symmetry center in order to achieve the luminescence mechanism using
friction. The term "structure having symmetry center" indicates
structures such as a perovskite structure, spinel structure, and
columbite structure. Details of the luminescence mechanism of the
friction-luminescent material will be given in FIG. 2.

[0078]The friction-luminescent material of the present invention
preferably can perform electroluminescence and plasma luminescence, in
addition to the structure in which the base material has symmetric
center. Moreover, the material can preferably retain an electric
potential, i.e. the material preferably has high charge-retaining
property and long decay time. As shown in an Example below,
high-luminosity stress-stimulated luminescence (friction luminescence) is
achieved by electrogenicity by friction and electroluminescence. That is,
electroluminescence is achieved when the crystal of the material has
lattice effect. Since it is possible to simultaneously realize both the
luminescence mechanism using friction and the luminescence mechanism
using lattice defect, high-luminosity stress-stimulated luminescence is
achieved.

[0079]Specific examples of the friction-luminescent materials including a
structure having symmetric center include: ZnAl2O4:Mn,
ZnGa2O4:Eu, MgAl2O4:Ce, MgGa2O4:Mn, and
CaNo2O6:Tb. In particular, a material with a spinel structure
having a Zn--Al--O defect structure, such as ZnAl2O4:Mn, is
preferably used. As described above, the material with such a structure
can simultaneously achieve the electroluminescence mechanism using
friction, the plasma luminescence mechanism, and the luminescence
mechanism using lattice defect. Moreover, the material can also exert
high thermo-luminescence as described in Examples below.

[0080]That is, it has been known that light-storing materials have high
thermo-luminescence, and the thermo-luminescence thereof is peaked around
room temperatures. It is therefore clear that all types of light-storing
materials emit light in response to thermal generation by friction.

[0081]Manufacturing Method of Friction-Luminescent Material>

[0082]The manufacturing method of the friction-luminescent material of the
present invention is not particularly limited, and any publicly-known
methods can be suitably used. More specifically, materials with a certain
composition ratio are mixed in an oxidative atmosphere so that a host
crystal structure is formed. In the friction-luminescent material of the
present invention, the base material is preferably reduced in the
temperature range at which the lattice defect is formed. This makes it
possible to surely and efficiently manufacture a material with a spinel
structure having a Zn--Al--O defect structure.

[0084]Among the stress-stimulated luminescent materials of the present
invention, a material (hereinafter, this material will be referred to as
strain luminescence material for convenience), which emits intense light
on account of strain caused by elastic deformation such as compression
stress and tensile stress, is, as described above, a stress-stimulated
luminescent material which emits light by a luminescence mechanism using
at least one of a piezoelectric effect, lattice defect, and thermal
generation on account of strain.

[0085]Luminescence Mechanism Using Piezoelectric Effect>

[0086]Although not particularly limited, the strain luminescent material
of the present invention is arranged such that the base material includes
a crystal structure including spontaneous polarization, in order to
realize the luminescence mechanism using a piezoelectric effect by
strain. More specifically, an example of the base material is
α-SrAl2O4.

[0087]It has been known that materials represented as
xSrO.yAl2O3 (e.g. Sr3Al2O6,
Sr2Al6O11, SrAl4O7, and
Sr4Al14O25) emits light in response to stress,
irrespective of the composition. It is found that these materials
actually shares a structure of SrAl2O4.

[0088]The α-SrAl2O4 is mono-crystal, and the crystal phase
thereof has spontaneous polarization. As described in Examples below,
crystal structures which are not mono-crystal does not have spontaneous
polarization. A crystal phase having spontaneous polarization performs
stress-stimulated luminescence in an elastic range.

[0089]Since the crystal phase, with spontaneous polarization is
ferroelectric, the strain luminescent material which performs
stress-stimulated luminescence in an elastic range is also ferroelectric.
The stress-stimulated luminescence in this case is carried out in such a
manner that strain energy by piezoelectricity is converted to
electricity, and light emission is performed by electroluminescence.
Therefore, the strain luminescent material is an electroluminescent
material, with a synergic effect of piezoelectric and
electroluminescence. That is to say, a strain luminescence material which
realize the luminescence mechanism using a piezoelectric effect by strain
is ferroelectric, and is an electroluminescent material.

[0090]As described above, regarding the strain luminescent material, it is
important that the symmetry center does not exist in the crystal
structure. Therefore, apart from α-SrAl2O4, it is
possible to effectively use materials in which symmetry center does not
exist in the crystal structure, for the base material of the present
invention. For example, many types of materials can be used apart from
aluminate. Examples of such materials include tungstate, niobate, and
titanate.

[0091]Luminescence Mechanism Using Lattice Defect>

[0092]Apart form the luminescence mechanism using a piezoelectric effect
by strain, as described above, the strain luminescent material of the
present invention may adopt a luminescence mechanism using lattice
defect. More specifically, for example, at least one type of metal ions
is added to the base material, as central ions of the defect center. In
particular, in case where the base material includes a crystal structure
with spontaneous polarization, the lattice defect is formed in the
crystal structure with spontaneous polarization in the base material, by
adding the aforesaid central ions.

[0093]For example, in case where, for example, the base material is
α-SrAl2O4, the structure of α-SrAl2O4 is,
as shown in FIG. 1, arranged such that the frame is formed by 6
tetrahedrons of AlO4, and Sr is provided in the tunnel. The central
ions are added in such a manner as to substitute Sr or Al sites. As a
result of this, the lattice defect is formed in the crystal structure
with spontaneous polarization. In this manner, the present invention
prefers, as the base material, a material in which the crystal structure
has a tunnel structure, and an element placed in the tunnel is preferably
a material placed by ionic bond.

[0094]In case where the added central ions substitute the Sr sites of
α-SrAl2O4, there are two cases: metal ions added as the
central ions are smaller in diameter than the Sr; and those metal ions
are larger in diameter than the Sr. In this manner, the luminous
intensity in the elastic range is further improved by adding metal ions
whose diameter is different from Sr in order to cause the crystal
structure to be easily strained.

[0095]The metal ions which are smaller in diameter than the Sr are not
particularly limited, and hence any metal ions of I-VIII group can be
used. Since the diameter of Sr2+ ions is 0.132 nm, metal ions whose
diameter is shorter than this are selected. Specific examples of such
ions are as follows: Ca, Mg, Na, Ti, Zr, V, Nb, Ta, Cr, Mn, Co, Ni, Sn,
Cu, Zn, Y, Cd, Mo, Ta, W, Fe, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb, and Lu. One of these metal ions is used or more than one of these
metal ions are used in combination.

[0096]Among these ions, at least one ion selected from the group
constituted by Mg, Na, Zn, Cu, Eu, Tm, Ho, Dy, Sn, Mn, Nd, Pr, and Ca is
preferably used. Using the metal ions selected from this group, it is
possible to cause the crystal structure to be easily strained, thereby
improving the luminous intensity in the elastic range.

[0097]The metal ions which are larger in diameter than the Sr are not
particularly limited, either. Specific examples of such ions include Ba,
K, and Pb. Ba and/or K is particularly preferable. One of these metal
ions is used or more than one of these metal ions are used in
combination.

[0098]The Sr sites are preferably substituted by both types of ions which
are larger and smaller in diameter than the Sr. This further facilitates
the strain of the crystal structure, and hence the luminous intensity in
the elastic range is improved.

[0099]In the above-described stress-stimulated luminescent material, at
least two types of metal ions having different valency are preferably
added as metal ions added as the central ions. For example, when the
metal ions are Sr as above, since Sr is positive bivalent ions
(Sr+2), it is preferable to add metal ions with different valency,
i.e. positive univalent, positive tervalent, positive quadrivalent,
positive quinquevalent or positive sexivalent metal ions. This is
advantageous in that lattice defect is effectively formed.

[0100]As described above, in the case of the example of
α-SrAl2O4, the structure has a high degree of freedom
because Sr sites are structurally in the tunnel in frame formed by
AlO4. As a result, as described above, the Sr sites can be
substituted by various types of metal ions. The same goes for cases where
base material is different from α-SrAl2O4. In this
manner, the tunnel structure and an easy-to-strain structure are
advantageous for stress-stimulated luminescence.

[0101]An amount of metal ions added as the central ions and substituting
the Sr sites of α-SrAl2O4 are not particularly limited,
on condition that the crystal structure of α-SrAl2O4 is
maintained. The amount is preferably in the range of 0.1-40 mol % of Sr.
Provided that the amount of metal ions falls within the aforesaid range,
the crystal structure is maintained but easily strained. This makes it
possible to effectively improve the luminous intensity in the elastic
range.

[0102]In case where metal ions smaller in diameter than the Sr and metal
ions larger in diameter than the Sr are both added as the central ions,
the total amount of the metal ions is preferably smaller than
stoichiometry. With this, it is possible to further improve the luminous
intensity in the elastic range.

[0103]In case where the added central ions substitute Al sites of
α-SrAl2O4, there are two cases: metal ions added as the
central ions are smaller in diameter than Al; and the metal ions are
larger in diameter than Al. The Al sites are tetrahedrons of AlO4.
On this account, there are not many choices of substituting metal ions,
as compared to Sr sites.

[0104]The diameter of Al3+ ions is 0.053 nm. Examples of metal ions
smaller in diameter than the Al3+ ions are B and Si, and B is
preferable. The luminous intensity is further improved by adding B. On
the other hand, examples of metal ions which are larger in diameter than
Al ions are Ga, In, Tl, Zr, Ti, V, and Nb. Among these metal ions, Ga and
In are preferable.

[0105]Being similar to the metal ions substituting the Sr sites, the metal
ions substituting Al sites are preferably both smaller and larger in
diameter than Al. The amount of the metal ions substituting the Al sites
is not particularly limited, on condition that the crystal structure of
α-SrAl2O4 is maintained. The amount is preferable 0.1-20
mol % of Al.

[0106]Luminescence Mechanism Using Thermal Generation>

[0107]In addition to the luminescence mechanisms using a piezoelectric
effect by strain and using lattice defect, the strain luminescent
material of the present invention may adopt a luminescence mechanism
using thermal generation. A friction-luminescent material may have this
luminescence mechanism. More specifically, for example, all types of
light-storing materials can emit light in response to thermal generation
by friction.

[0108]Manufacturing Method of Strain Luminescent Material>

[0109]A manufacturing method of the strain luminescent material of the
present invention is not particularly limited. Any publicly-known
manufacturing methods are used in such a manner that the crystal
structure with which the aforesaid luminescence mechanism is realized is
formed. More specifically, in the case of α-SrAl2O4, the
raw materials are mixed and burned in such a manner as to form a crystal
structure with spontaneous polarization in the base material. Examples of
the raw materials are SrCO3 and Al2O3. To add metal ions
which function as central ions of the defect center, oxides of the metal
ions are mixed into the material or the material which eventually becomes
oxide. In Examples below, Eu2O3 is added because the central
ions are Eu. The present invention, however, is not limited to this case.

[0110]The conditions of the burning are not particularly limited.
Predetermined amounts of materials are mixed so that a desired ratio of
ions are included, and the burning is carried out with publicly-known
burning conditions. In Examples below, the burning step is carried out
after tentative burning in the air. This makes it possible to achieve an
optimum crystal structure and lattice defect level. The burning step is
carried out in any types of inactive gas atmosphere. To the atmosphere, a
predetermined amount of gas such as H2 may be added according to
need. The burning temperature falls in the range of 1000-1700° C.
in Examples below, but not necessarily in this range. The burning can be
carried out in a publicly-known temperature range, on condition that the
crystal structure is sufficiently formed.

[0111](III) Use of Present Invention

[0112]The use of the present invention is not limited to any particular
field. The present invention can be used in any fields where
stress-stimulated luminescence of the above-described stress-stimulated
luminescent material is performed. For example, the stress-stimulated
luminescent material is manufactured to have a specific form and used as
a stress-stimulated luminescent body.

[0113](III-1) Stress-Stimulated Luminescent Body

[0114]The specific arrangement of the stress-stimulated luminescent body
is not particularly limited. Examples of the arrangement of the
stress-stimulated luminescent body are (1) powder and sintered body, (2)
molded article in which the stress-stimulated luminescent body is mixed
with another material, and (3) the stress-stimulated luminescent body is
applied onto the surface of a supporting material. In the case of the
powder and sintered body, the stress-stimulated luminescent material of
the present invention is used almost without any alterations. The
particle diameter and particle size distribution of the powder and the
shape and size of the sintered body are not particularly limited.

[0115]In case where the stress-stimulated luminescent material of the
present invention is mixed with another material and molded, for example,
when the luminescence mechanism using strain is adopted, the
stress-stimulated luminescent material is mixed with a polymeric material
and molded into a flat plate. A non-limiting examples of the material
mixed with the stress-stimulated luminescent material is epoxy resin. The
amount of the material to be mixed is not particularly limited, on
condition that the stress-stimulated luminescent material can be molded
into a flat plate and the shape can be maintained. In Examples below, the
weight ratio is 1:1. The conditions at the time of the mixture are not
particularly limited, either. Publicly known methods can be used.

[0116]In case where the stress-stimulated luminescent material is applied
to the surface of the supporting material, for example, a laminated
structure in which a layer of the stress-stimulated luminescent material
is provided on the supporting body may be used. When a laminated
structure is formed, non-limiting examples of the supporting body on
which the stress-stimulated luminescent material layer is provided are
metal, fiber, rubber, fabric, polymeric material, paper, glass, and
ceramics. When luminescence is achieved by low strain energy, a material
with a low elastic coefficient is preferable. If the elastic coefficient
is too low, the variation speed of the strain is low and hence the strain
energy is low. As a result the luminous efficiency is not good and the
luminous intensity is low if compared with a case where a material is
under a certain identical stress. For this reason, it is undesirable that
the elastic coefficient is too low. Examples of the supporting material
satisfying the requirement above are synthetic resins such as acrylic
resin and epoxy resin and elastomeric materials such as paper and rubber.
In particular, as described in Example, paper is preferable because very
bright visible light is emitted in response to touch by a finger, even if
external light is intense such as sunlight.

[0117]Even if the elastic coefficient is low, the stress-stimulated
luminescence is not so high in the case of materials which has high
stress relaxation e.g. elastomeric materials such as rubber. This is
presumably because the variation speed of the stress is slowed. To put it
differently, the luminous intensity is changed when the supporting
material is an elastomeric material, even if the stress to be applied is
not changed. The supporting material is therefore suitably selected in
consideration of the use and desired luminous intensity.

[0118]Any types of methods can be used for providing the stress-stimulated
luminescent material of the present invention on the supporting body. The
material in the form of paste or paint may be applied using a
publicly-known method, or a film-shaped material may be provided on the
surface of the supporting body. The thickness of the applied
stress-stimulated luminescent material is not particularly limited,
either. Typically the thickness preferably falls within the range of
1-1000 μm, and more preferably falls within the range of 10-500 μm.
When the thickness is in these ranges, the stress-stimulated luminescent
material is efficiently strained by applying stress.

[0119]To provide the stress-stimulated luminescent material on the
supporting body, a publicly-known adhesive may be used. In particular,
the durability of the stress-stimulated luminescent body is improved when
an adhesive which has an adhesive strength corresponding to the
supporting body is selected. This makes it possible to effectively avoid
the cracking and breaking of the stress-stimulated luminescent body on
account of repetitive or continuous application of stress. Non-limiting
and preferable examples of the adhesive are silicon, polyimide, starch,
polyvinyl-vinyl acetate copolymer, epoxy resin, polyamide resin, and
cyanoacrylate adhesives. Such adhesives are preferable because of low
stress relaxation as compared to other types of adhesives and a high
adhesion force.

[0120]The shape of the stress-stimulated luminescent body with a laminated
structure is not particularly limited, and the stress-stimulated
luminescent body may have any shapes on condition that a deforming force
is efficiently applied. The stress-stimulated luminescent body is
preferably thin-film-shaped or film-shaped, and more preferably has a
diaphragm structure. The diaphragm structure is a bulkhead structure
using a flexible film, and used for a variety of uses such as detecting
the variation of pressure and generating displacement. In the present
invention, the diaphragm structure is preferable because an applied
deforming force is effectively used for stress-stimulated luminescence.

[0121]For example, the diaphragm structure is constructed using the bottom
of a paper cup, in Example below. The stress-stimulated luminescent
material of the present invention is provided on the bottom of the paper
cup (i.e. paper as the supporting body), so that the stress-stimulated
luminescent body with the diaphragm structure is formed.

[0122](III-2) Luminescence Method

[0123]The use of the present invention includes a luminescence method of
the stress-stimulated luminescent material of the present invention.
Since the luminescence method is not particularly limited, a suitable
method is selected in consideration of the luminescence mechanism of the
stress-stimulated luminescent material.

[0124]Luminescence Mechanism Using Friction>

[0125]Concretely speaking, in the case of a stress-stimulated luminescent
material adopting the luminance mechanism using friction, for example,
luminescence is achieved by rubbing, using an optional friction material,
the stress-stimulated luminescent material or a stress-stimulated
luminescent body which is made of the stress-stimulated luminescent
material and has a suitable shape.

[0126]A non-limiting and suitable example of the friction material is a
material which excels in an electrostatic force (electromotive force) by
friction, micro plasma by friction, recombination by friction of
electrons and positive holes trapped in lattice defect, or
thermo-luminescence by friction. In other words, according to the
luminescence mechanism using friction, the electromotive force, micro
plasma, recombination of electrons and positive holes, and
thermo-luminescence depend on the type of the friction material cause
thermo-luminescence. The friction material is therefore selected in
accordance with a desired luminescence level.

[0127]Since high luminous intensity is basically preferable, a friction
material which is suitable for the above-described four types of
luminescence mechanisms is preferable. Although the friction material is
not particularly limited, the friction material preferably has high
electric resistance. More specifically, the friction material has volume
resistivity (25° C., 50% RH) of not less than 1014 Ωcm,
more preferably has electric resistance of not less than 1016
Ωcm. A preferable example of such a material is polyethylene. More
specifically, in the case of a base material with a spinel structure
having a Zn--Al--O defect structure, as described in Examples below,
polyethylene is preferable as the friction material, because the material
is suited to the aforesaid four types of luminescence mechanisms. The
surface of the friction material is preferably antiwear-coated.

[0128]Luminescence Mechanism Using Deforming Force>

[0129]In the case of the luminescence mechanism using a piezoelectric
effect or lattice defect (or thermal generation), i.e. in the case of the
stress-stimulated luminescent material adopting the luminescence
mechanism using deforming force, luminescence is achieved by applying an
external force to a flat-plate-shaped stress-stimulated luminescent body
or a stress-stimulated luminescent body having a laminated structure.

[0130]It has been know that, in a stress-stimulated luminescent body with
a structure such as lattice defect, the stress-stimulated luminescence
conspicuously peaks after the application of intense ultraviolet light or
radiation, and then the stress-stimulated luminescence is kept at a
certain level after attenuation. On this account, according to the
luminescence method of the stress-stimulated luminescent material of the
present invention, ultraviolet light may be applied to the
stress-stimulated luminescence material while an external force is
applied thereto. As a result of this, it was found that the
stress-stimulated luminescence without attenuation was achieved. The
irradiation conditions of ultraviolet light and radiation are not
particularly limited. The conditions suitable for effective
stress-stimulated luminescence are selected. For example, since the
irradiation time is not particularly limited, ultraviolet light may be
applied for a certain length of time or applied as pulses. The
irradiation intensity is not particularly limited, either, and may be
determined in line with publicly-known methods. However, the wavelength
identical with luminescence length is not used, because the measurement
of the luminescence becomes difficult.

[0131](III-3) Specific Applications of Present invention

[0132]In the present invention, the aforesaid luminescence method of the
stress-stimulated luminescent body and the stress-stimulated luminescent
material is used so that the stress-stimulated luminescent material of
the present invention can be used for concrete technical fields. For
example, the present invention can be used for developing new pressure
sensors and vital function measuring sensors such as a pressure sensor
operable in a severe temperature environment and a sheet-shaped pressure
sensor which is thin and flexible. Also, using the stress-stimulated
luminescent material of the present invention as a sensing material and
integrating the material with a publicly-known system or a system to be
developed in future, it is possible to create a new field of sensing. In
particular, present invention realizes direct visualization, and
measurement at two or three dimensions.

EXAMPLES

[0133]The present invention will be specifically described by Examples.
The present invention is not limited to these examples, and may be varied
in many ways. Such variations are not to be regarded as a departure from
the spirit and scope of the invention, and all such modifications as
would be obvious to one skilled in the art are intended to be included
within the scope of the following claims.

Example 1

[0134]Among the stress-stimulated luminescent materials of the present
invention, a material adopting the luminescence mechanism using friction
will be specifically described in reference to FIGS. 3-12.

[0135]ZnO, Al2O3, and MnCO3 were weighed so as to satisfy
Zn0.95Mn0.05Al2O4, and sufficiently mixed in ethanol.
Then the mixed substances were dried and crushed. Subsequently, the
substances were burned in the air at 1250° C. and for 8 hours, and
a sample was obtained as a result. Analyzing the crystal structure of the
sample using an X-ray diffractometer, it was found that a material with a
pure spinel structure was obtained as shown in FIG. 3(a).

[0136]Thereafter, the obtained material (ZnAl2O4:Mn) with the
spinel structure was reduced in 5% H2--Ar, at temperatures of
300-1300° C. FIG. 4 shows the weight loss curve during the
reduction. The weight loss at temperatures not higher than 600° C.
was caused by the desorption of adsorbed water and oxygen. The weight
rapidly decreased at 1200° C., because of the desorption of Zn and
O in the spinel crystal. As a result of this, it was found that the
reduction at temperatures at which the lattice defect was generated was
effective for manufacturing a material with the spinel structure having
the defect structure.

[0137]FIG. 5 shows a graph of the fluorescence intensity at the time of
the reduction. As the graph clearly shows, the fluorescence intensity of
the material with the spiel structure having the defect structure started
to decrease when the temperature exceeds 1200° C.

[0138]An experimentation about thermo-luminescence of above-described
ZnAl2O4:Mn was carried out using a fluorescence spectral
photometer, with temperature increase at the rate of 10° C./min.
The result of this experimentation is shown in FIG. 6. As the result
clarifies, the thermo-luminescence of the material with the spinel
structure having the defect structure was the highest as compared to
materials with other structures.

[0139]As samples for a friction experimentation, two types of pellets were
used: a pellet of ZnAl2O4:Mn ceramics and a pellet in which
ZnAl2O4:Mn powder is mixed with epoxy resin. The mixed pellet
was obtained by mixing a friction-luminescent material
(ZnAl2O4:Mn) of 1 g with epoxy resin of 3.5 g and molded into a
pellet of Φ25×15 mm. The friction experimentation was carried
out as follows: the friction experimentation samples were fixed on a
sample table of a friction testing machine, and the samples were rubbed
at a predetermined load (2N) and a predetermined experimentation speed
(2.8 cm/s). The friction luminescence was measured using a photo counter,
so that the reduction temperature dependency was evaluated. The result of
this experimentation is shown in the graph in FIG. 7. As the graph
clearly illustrates, the intensity of the thermo-luminescence of the
material with the spinel structure having the defect structure started to
decrease when the temperature exceeded 1200° C. Also, the
reduction dependency of the friction luminous intensity and the response
curve of the friction luminescence were evaluated, as shown in the graph
in FIG. 8. It was found that the material with the spinel structure
having the defect structure exhibited high friction luminescence.

[0140]Regarding ZnAl2O4:Mn, the spectrums in the cases of
fluorescence (PL), thermo-luminescence (ThL), and friction luminescence
(ML) were measured using a multi-channel spectrometer. FIG. 9 shows the
results of the measurements. As clearly shown in the figure, it was found
that the observed friction luminescence was caused not by N2 gas
discharge but by the central ions (Mn) in ZnAl2O4:Mn, because
the luminescence did not peak around the range of 380-480 nm at which
luminescence by N2 gas discharge is typically peaked.

[0141]Thereafter, ultraviolet light is irradiated for one hour by a UV
lamp with 365 nm, and then afterglow and friction luminescence were
measured in a dark room. By doing so, the correlativity between the
afterglow intensity and the stress-stimulated luminescence (friction
luminescence) was measured. FIGS. 10(a) and 10(b) shows the results. FIG.
10(a) shows the correlativity between the reduction temperature and the
friction luminous intensity. FIG. 10(b) shows the relationship between
the afterglow intensity and the friction luminous intensity, regarding
ZnAl2O4:Mn having been subjected to the reduction at
1200° C. According to these results, it was found that the
friction luminescence was kept for a long time even if the afterglow
decreased.

[0142]Regarding the aforesaid ZnAl2O4:Mn, the correlativity of
the friction luminescence and the surface potential was then analyzed, by
measuring the surface potential by using a surface electrometer
concurrently with the measurement of the friction luminescence. The
results are shown in FIGS. 11(a) and 11(b). FIG. 11(a) shows the
intensity of the friction luminescence of each friction material. FIG.
11(b) shows the surface potential of each friction material. As the
results clearly show, the luminescence at the time of rubbing the
ZnAl2O4:Mn by the friction material was caused by the surface
potential generated by the friction. It was also found that the luminous
intensity was high when an electrostatic force caused by friction was
high. The experimentation proved that polycarbonate and polyethylene were
preferable to brass and polyether ether ketone. Polyethylene was
particularly preferable.

[0143]Then, as a sample for electroluminescence, a powdered
spinel-structured material (Zn--Al--O) with a defect structure was molded
into a pellet of Φ20×1 mm, at a hydrostatic pressure of 2
t/cm2. The sample was then reduced in 5% H2--Ar. On the both
surfaces of the obtained defected-spinel-structured ceramics, an Al
electrode and an ITO transparent electrode were formed. Using such a
sample, the electroluminescent spectrum was measured by a fluorescence
spectral photometer, while a DC voltage of 1.5V was applied to the
sample. The result of this measurement is shown in FIG. 12. Since the
result shows that there was a peak around 520 nm, the material with the
spinel structure having the defect structure had high
electroluminescence.

Example 2

[0144]High purity reagents SrCO3, Al2O3, and
Eu2O3 were weighed so as to satisfy
Sr0.99Eu0.01Al12O19,
Sr0.99Eu0.01Al4O7,
Sr3.99Eu0.01Al14O25,
Sr0.99Eu0.01Al2O4, and
Eu0.01Sr2.99Al2O6, and were sufficiently mixed with
one another. Thereafter, the mixed reagents were tentatively burned at
800° C. in the air, and then the burning was carried out at a
temperature range of 1000-1700° C. and in the atmosphere of Ar+5%
H2. As a result, stress-stimulated luminescent bodies with the
aforesaid compositions were obtained. As a result of examining the
crystal structure of each of the obtained stress-stimulated luminescent
bodies by X-ray diffraction, it was found that the crystal structure was
pure.

[0145]The obtained particulate (powder) stress-stimulated luminescence
material was mixed with epoxy resin at a weight ratio of 1:1, and a
rectangular-parallelepiped sample (i.e. a stress-stimulated luminescent
body) of 54 mm, 19 mm, and 5 mm in length, width and height was formed.

[0146]Subsequently, as shown in FIG. 13, the luminous intensity, stress
(deforming force), and strain were simultaneously measured while a stress
was applied at a constant speed. The measurement was carried out using a
measurement system including an image taking section 10, a spectrum
measurement section 20, a luminous intensity measurement section 30, and
a sample fixing section 40. In this measurement system shown in FIG. 13,
the image taking section 10 is constituted by an ICCD camera 11 and a
computer 12, the spectrum measurement section 20 is constituted by a
multi-channel spectrum analyzer 21 and a computer 22, and the luminous
intensity measurement section 30 is constituted by a PM (photomultiplier)
31, a photon counter 32, and a computer 33. It is noted that the image
taking section 10, the spectrum measurement section 20, and the luminous
intensity measurement section 30 are not necessarily arranged in this
way. The multi-channel spectrum analyzer 21 is connected to a condensing
lens 24 via a glass fiber 23, whereas the PM 31 is connected to a
condensing lens 35 via a glass fiber 34. Each of these devices measures
luminous intensity or the like.

[0147]The rectangular-parallelepiped sample 50 was fixed to a sample
fixing section 40 so that a strain gauge 41 was provided on the crosswise
surface (of 19.2 mm×7.5 mm). A stress was applied to the sample 50
by longitudinally (along the side of 54 mm) applying load by a loading
cell 42. The entire surfaces of the sample were pictured by the ICCD
camera of the image taking section, and the luminous intensity or the
like was measured using measurement probes connected to, by glass fiber,
the spectrum measurement section and the luminous intensity measurement
section. While the experimentation, the strain forming speed by the
stress application was varied. Table 1 shows the obtained luminous
intensity in the elastic range.

[0148]As the results in Table. 1 clearly show, stress-stimulated
luminescence in the elastic range was not observed in crystals such as
Sr3Al2O6, which was a known stress-stimulated luminescent
material.

[0149]As shown in Table. 1, only α-SrAl2O4 exhibited
stress-stimulated luminescence in the elastic range. The crystal
structure of this substance was monoclinic crystal. Spontaneous
polarization occurs in this crystal phase, but does not occur in other
crystal structures. On this account, stress-stimulated luminescence in
the elastic range is exhibited by a crystal phase with spontaneous
polarization. As a result of material development experiments over a long
period of time, it has been proved that the rule above is applicable to
other stress-stimulated luminescent bodies. Also, since a crystal phase
with spontaneous polarization exhibits ferroelectricity, a
stress-stimulated luminescent body in the elastic range has
ferroelectricity. The luminescence is achieved in such a manner that
strain energy is converted into electricity, and light is emitted by
electroluminescence. As described above, a stress-stimulated luminescent
material which emits light in response to stress in the elastic range has
ferroelectricity and is electroluminescent.

Example 3

[0150]In the same manner as Example 2, SrCO3, Eu2O3, and
Al2O3 were mixed so as to have various compositions. As a
result, a crystal phase coexisting with the α-SrAl2O4
phase was synthesized. FIGS. 14(a) and 14(b) shows the variations of
stress-stimulated luminous intensities in case where the
α-SrAl2O4 phase coexisted with a non-luminous crystal
phase. The result proved that, even if stress-stimulated luminescence was
achieved because of the coexistence of the α-SrAl2O4
phase, the luminous intensity was low. On the other hand, the highest
luminous intensity was achieved when only the α-SrAl2O4
phase was provided. As shown in FIGS. 15 and 16, the luminous intensity
in the case of the coexistence changed in accordance with the variation
of the stress, and the luminescent spectrum was peaked at 520 nm, as in
the case where only the α-SrAl2O4 phase was provided.

Example 4

[0151]Examples 2 and 3 confirmed that the highest luminous intensity was
obtained in the case where only the α-SrAl2O4 phase was
provided. Taking into account of this, it was assumed that forming
lattice defect in a crystal structure was advantageous for further
facilitating the strain. On this account, a lattice-defect-control
sample, in which an amount of Sr2+ ions was smaller than
stoichiometry so that lattice defect was formed in Sr sites, was formed.
As a result, as shown in FIG. 17, the luminous intensity was dramatically
improved when an amount of lattice defect was optimal. When the amount of
the defect was too small, the effect of the lattice defect was
insufficient and the luminous intensity was low. On the other hand, when
the amount of the defect was too large, the crystal structure was broken
and hence the luminous intensity was also low. It was found that the
luminous intensity was significantly improved by providing lattice defect
while a pure-phase structure is maintained, i.e. only the
α-SrAl2O4 phase was provided.

Example 5

[0152]To further facilitate strain and improve luminescence, it was
assumed to be advantageous to add, to Sr2+, at least one type of:
ions having higher valence than the Sr2+; ions having lower valence
than the Sr2+; ions which are long in diameter; and ions which are
short in diameter. On this account, using high purity reagents,
stress-stimulated luminescent materials with different compositions were
manufactured in the similar manner as Example 2, by weighing SrCO3,
Eu2O3, Al2O3, KI, NaI, CaCO3, BaCO3,
B2O3, MgCO3, Ho2O3 so as to achieve
predetermined compositions and sufficiently mixing them. Table. 2 shows
some of the obtained stress-stimulated luminescent materials exhibiting
high luminous intensity.

[0153]The crystal structure of each stress-stimulated luminescent material
was identified by X-ray diffraction. As a result, it was confirmed that
all stress-stimulated luminescent materials had the
α-SrAl2O4 phase. Among stress-stimulated luminescent
materials in which lattice defects were formed in both Sr sites and Al
sites, the highest stress-stimulated luminescence in the elastic range
was exhibited by a stress-stimulated luminescent material in which more
than two types of ions having different valence were added to both types
of sites.

[0154]FIG. 18 shows an example of a thermo-luminescence chart of the
material exhibited high stress-stimulated luminous intensity. According
to this figure, it was found that there were plural peaks of
thermo-luminescence, and luminescence was exhibited in a wide temperature
range. This was because plural lattice defects were formed. Also, since
thermo-luminescence was observed around room temperatures, the
thermo-luminescence on account of strain by deformation (i.e.
luminescence mechanism using thermal generation) was exhibited
concurrently with the luminescence mechanism using lattice defect. In
other words, plural luminescence mechanisms using strains were
concurrently used, so that the luminescence was further improved.

Example 6

[0155]A stress-stimulated luminescent material (powder) with the
composition
(Sr0.60K0.02Ho0.02Mg0.10Ba0.15)(Al1.95B.sub-
.0.05)O4 was mixed with epoxy resin at a weight ration of 1:1, and
the luminous intensity, stress, and strain were simultaneously measured
while a stress is applied, using a measurement system shown in FIG. 13,
in a similar manner as Example 2. The results of this are shown in FIGS.
19, 20, and 21.

[0156]FIG. 19 shows the variation curve of the luminous intensity when a
stress was applied. As the stress increased, the luminescence linearly
increased. In this manner, plotting peak values of stress-stimulated
luminescence and stress, which were obtained by experimentations with
stresses at various peaks, it was found that these peak values were
linearly in proportion to each other as shown in FIG. 20. (In the figure,
ε=0.3×10-3 l/s) Also, plotting the relationship
between stress-stimulated luminescence values and strain speeds at the
time experimentations with various strain speeds, it was found that these
values were in proportion to each other. (In the figure,
σmax=2.28 MPa)

[0157]Regarding the stress-stimulated luminous intensity from a start
period to a period t, values figured out by integration by time proved
that, as indicated by the following equation (1), the variation of the
strain energy density from the start period to the period t is
proportional.

[0159]In the elastic range, as shown in the following equation (2), the
variation was in proportion to a value figured out by integrating the
product of the stress and the strain by time. In the equation, the
proportion coefficient was k/2.

[0161]The equation (2) was differentiated. As a result, the relationship
indicated by the following equation (3) was given.

I(t)-I(t0)=kσ(t){dot over (ε)}(t) (3)

[0162](where ε indicates strain speed, which is in proportion to
the rate of change of stress and in inverse proportion to the elastic
coefficient)

[0163]A value figured out by multiplying the proportion coefficient k by
the product of the stress at an optional time t and the strain speed at
the time t was equal to the difference between the stress-stimulated
luminous intensity at the time t and the stress-stimulated luminous
intensity at the start time t0. The result shown in the equation (3)
was in agreement with the experimentation results shown in FIGS. 20 and
21, and the stress-stimulated luminous intensity was in proportion to
both the stress and strain speed. Therefore, the stress-stimulated
luminous intensity is in proportion to strain energy, and also in
proportion to the stress distribution when the strain speed is constant.
Therefore, when the stress-stimulated luminescent material is applied to
the object, remote monitoring is feasible because the stress distribution
of the object is in proportion to the luminescence image.

[0164]According to the equation (3), the important points for an effective
luminescence method to obtain high stress-stimulated luminescence are the
following two:

[0165]Select an object having a low elastic coefficient; and

[0166]Select a structure having no stress relaxation.

Example 7

[0167]A paste obtained by mixing a stress-stimulated luminescent material
powder with epoxy resin was applied onto various sample pieces
(40×3×200 mm) so that flat plates (films) which were 10
μm, 50 μm, and 100 μm thick were formed on each of the sample
pieces. Using the device shown in FIG. 13 and a tensile jig, a stress
cycle was repeatedly exerted onto the sample pieces. With these
conditions, the correlativity between stress and stress-stimulated
luminescence was measured. Table. 3 shows a comparison between
stress-stimulated luminous intensities of stress-stimulated luminescent
films which were 50 μm thick when a stress cycle with a frequency of 1
Hz was applied.

[0168]As the result clearly shows, the luminescence at a constant stress
increased as the elastic coefficient of the sample piece decreased.
However, as to a material such as rubber which has high stress
relaxation, the stress-stimulated luminescence was not very high even if
the elastic coefficient was low. This was presumably because the
variation speed of the stress was relaxed. Also, it turned out to be very
effective to introduce an adhesive suitable for the sample piece, because
cracks and breakings of the stress-stimulated luminescent film were not
observed at all in the experimentation. As the adhesive,
commercially-available cyanoacrylate adhesive and high-density epoxy
adhesive were particularly effective. This was presumably because these
adhesives had low stress relaxation as compared to other types of
adhesives.

[0169]When the stress-stimulated luminescent material was molded into a
thin film, the stress-stimulated luminescent intensity was high, while
the film was easily strained but not easily broken. It was also found
that the supporting body of the film was preferably made of a material
whose elastic coefficient is similar to that of the film.

Example 8

[0170]With the conditions in Example 7, a case where ultraviolet light
irradiation was performed was compared with a case where ultraviolet
light irradiation was not performed, in regard to the variation of the
stress-stimulated luminescence peak when a stress cycle with a frequency
of 1 Hz was applied. As to the conditions of the ultraviolet light
irradiation, ultraviolet light with 365 nm was irradiated for one minute.
As a result, as shown in FIG. 22, in response to the repeated stresses,
attenuation occurred before a stable state, and the attenuated
stress-stimulated luminescence was regained by the irradiation of the
ultraviolet light. Also, the variation of the stress-stimulated
luminescence peak when a stress cycle with a frequency of 1 Hz was
applied while ultraviolet light was applied was evaluated. As a result,
as shown in FIG. 23, the variation was stable in response to the repeated
stresses.

Example 9

[0171]A stress-stimulated luminescent body having a laminated structure
was manufactured by providing a stress-stimulated luminescent material
about 10-μm thick on the bottom of a commercially-available paper cup.
Very bright light was observed even in daylight, when the bottom (outer
surface) of the paper cup was slightly pushed by a finger. The luminous
intensity was at least 10 cd/m2. This proved that a diaphragm
structure with which strain was easily applied was tremendously effective
as a method of effectively causing a stress-stimulated luminescent
material to emit light.

[0172]The invention being thus described, it will be obvious that the same
way may be varied in many ways. Such variations are not to be regarded as
a departure from the spirit and scope of the invention, and all such
modifications as would be obvious to one skilled in the art are intended
to be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

[0173]As described above, by the present invention, 5 basic luminescence
mechanism for obtaining a high-luminosity stress-stimulated luminescent
material were clarified, and based on this, a stress-stimulated
luminescent material which can realize high luminous intensity and a
method of use thereof were developed. As a result of this, it is possible
to effectively obtain an excellent stress-stimulated luminescent material
which has characteristics such as high intensity, long afterglow, long
response time and the like, as compared to conventional materials.

[0174]As described above, since a luminescent mechanism was clarified for
the first time, the present invention makes it possible to effectively
obtain a stress-stimulated luminescent material whose luminous intensity
is higher than conventional materials. The present invention can
therefore be suitably used for a material production industry such as
manufacturers of stress-stimulated luminescent materials and/or
stress-stimulated luminescent bodies, and also for novel sensing
technologies. On this account, the present invention is applicable to
industrial fields such as electronic components and electronic/optical
devices, e.g. manufacturing sensors, and also to various industrial
fields such as safety management, measurement, robot, toy, and the like.